1. Field of the Invention
The present invention concerns hydraulically controlled clutch release bearings for a diaphragm clutch, notably for a motor vehicle, having a fixed portion comprising an inner support tube and an outer body concentric therewith, defining a blind annular cavity of axial orientation capable of being supplied with fluid and inside which is mounted an axially movable tubular piston.
1. Description of the Related Art
Such a bearing is described for example in the document FR-A-2 692 950. In this document, the inner tube projects axially in relation to the outer body.
The piston carries at its front end a drive member capable of acting on the diaphragm of the clutch.
This bearing can be supplied by a fluid of the gaseous or liquid type and this is the reason why it is usually referred to as a hydraulically controlled clutch release bearing.
There is a problem with this type of bearing as impurities can enter the cavity defining with the piston a control chamber of variable volume.
The object of the present invention is to overcome this drawback in a simple and economical manner.
According to the invention, a bearing of the type indicated above is characterized in that the front end of the tube is surrounded by a guide sleeve for the piston.
Thus, by virtue of the invention, a baffle is created, preventing the entry of contaminants into the control chamber in a simple and economical manner.
Advantageously, this sleeve is made of a material with a low coefficient of friction and is attached with axial fixing to the tube, for example by crimping, snapping-in or adhesive bonding.
In one embodiment this sleeve has at least one catch capable of entering in a complementary manner into an indentation in the tube for its fixing to the said tube.
Advantageously, at least two catches and two indentations are provided.
This sleeve also makes it possible to limit the axial travel of the advantageously internally stepped piston so that the said piston has an internal shoulder capable of cooperating with the rear end of the guide sleeve.
Thus the piston remains inside the blind cavity notably before the bearing is associated with the clutch.
The fixed portion of the clutch release bearing can be in one piece but advantageously, in order to facilitate the installation of the piston inside the blind cavity and the insertion of the piston on the guide sleeve, the inner tube is separate from the outer body. This inner tube is metallic and has in one embodiment, at its rear end, an annular flange of transverse orientation allowing the outer body to be centred.
To this end, this flange has an annular rim of axial orientation for centring the guide tube in relation to a fixed portion, namely the casing of the gearbox in the case of an application for a motor vehicle.
By virtue of this embodiment, it is possible to have a thinner inner tube than in the case where the fixed portion is in one piece, with an inner tube obtained in an economical manner.
According to one characteristic the rear end of the piston carries a joint by means of a joining piece anchored in the said joint and mounted by snapping onto the rear end of the piston.
The joint enables the control chamber to be sealed and cannot come out of the blind cavity by virtue of the shoulder of piston cooperating in the aforementioned manner with the rear end of the guide sleeve.
The joint has lips and a pin capable of coming into abutment against the base of the cavity when the clutch is in the engaged position.
Noise is thus reduced, and the outer body serves to guide the joint so that it does not need to be internally machined in a precise manner, the guiding of the piston being effected by the guide sleeve and by the inner tube cooperating with the rear end of the piston.
Advantageously, the piston has at its front end a nose and an annular flange of transverse orientation enabling the mounting of a ball bearing capable of coming into contact with the inner end of the fingers of the clutch diaphragm. The guide sleeve provides a good support for the nose.
This nose of the piston, in one embodiment, has a groove for mounting an axially acting elastic washer making it possible to clamp a radial edge offered by the inner raceway of the bearing in contact with the flange of the piston.
A clutch release bearing of the self-centring type is thus obtained.
By virtue of all these arrangements it is easy to insert between the outer body and the flange of the piston a pre-loading spring permanently forcing the bearing into contact with the ends of the fingers of the diaphragm. By virtue of this pre-loading spring it is possible, before installing the clutch release bearing inside the clutch, to create a unitary assembly, the flange of the guide tube coming into contact with the outer body, whilst the piston comes into abutment against the rear end of the guide sleeve in the aforementioned manner.
By extending the inner raceway of the bearing radially outwards it is possible to install a protective bellows surrounding the pre-loading spring and the piston. Thus the control chamber is protected by the bellows and by the guide sleeve.
According to one characteristic for reducing the axial dimension, the pre-loading spring, the piston and a tubular portion of the body are housed below the inner raceway of the bearing.